There are many situations in which there is a need or desire to measure the rate of flow of a liquid or gas through a conduit. Because of the shape of the conduit, temperature differentials within the conduit and other factors, not all of the fluid may be flowing parallel to a centerline through the conduit. Rather the true flow direction at any specific location may be, and frequently is, at some angle relative to the centerline. Furthermore, the flow rate may not be constant during the measurement period. Indeed, there may be perturbations or an oscillatory component to the velocity. Consequently, placing a pitot tube or other measuring device parallel to the walls of the conduit will not yield accurate results. In my U.S. Pat. No. 5,509,313 I disclosed a method for positioning a probe so that the probe was aligned with the true flow direction. Although this method has provided more accurate data the results have been above the true velocity.
The federal government of the United States has set limits as to the amount of pollutants that an electric utility or other business may emit into the air. Typically, these emissions are determined from measurements of the flow rate of the stack gasses through the stack and an analysis of the stack gasses to determine the levels of pollutants which are present. If one knows the flow rate and has another monitor which measures the concentration of pollutants in a selected volume of fluid one can calculate the quantity of pollutants emitted over any selected time period.
The United States has additional regulatory requirements which now require many electric utilities to continuously measure emissions of specified pollutants on a mass per unit time basis. Additionally, the continuous monitors must be periodically tested to assure that they are functioning properly. When such tests are done one must use a "reference method" measuring device. If the reference method monitor shows the continuous monitor to be reading low, the continuous monitor must be recalibrated. Adoption of these rules has put a new importance upon the errors which occur both in continuous monitoring and in the periodic reference method verification tests. Such errors can be very costly to both the supplier of the monitor and the utility. The supplier is affected because the reference method can erroneously indicate that the monitor is not meeting the performance guarantee. The utility is affected because it may have been reporting the emission of more pollutants than actually occurred. The new regulations establish monetary value in the form of trading credits to a measured ton of SO.sub.2 emissions. If the reference method is in error, that error will directly cause an enormous high or low use of the utility's SO.sub.2 allowance and SO.sub.2 trading credits. The value of such emissions is such that for large utilities as much as $1,000,000 per percent error in measured emissions may result. Thus, there is a need for a method which can accurately determine the true flow rate or velocity of a fluid flowing through a conduit.
In performing the required reference method verification tests the technician typically uses a type S (also called S-type) pitot tube made to specific dimensions. At specified points the tester measures a differential pressure. The static pressure is also taken at selected points. The differential pressure reading is then used to compute flow rate. From the computed flow rate and an analysis of fluid samples taken from the conduit, the amount of pollutants from the stack is calculated.
The test procedure required by the United States Environmental Protection Agency (EPA) is set forth in 40 Code of Federal Regulations, Part 60, Appendix A, Methods 1 and 2. This method can be performed by manually positioning the probes at the specified locations in the stack or by using an automated probe such as is disclosed in my U.S. Pat. No. 5,440,217. Within the past year the EPA and others have done studies to determine the accuracy of various manual and automated procedures currently being used. The test results indicated that all procedures resulted in calculated emissions of pollutants that were higher than the actual emissions. Furthermore, some methods produced results that were much higher than were the results than other methods. Those who conducted the tests did not have any explanation for the differing results.
In my U.S. Pat. No. 5,509,313 I reported that the art has conventionally determined flow rate by taking several differential pressure readings averaging these readings and then finding the square root of that average. However, I found that a more accurate result can be determined by taking the square root of each reading. Although this is true, flow rates and corresponding calculated emissions using this technique also were observed to be higher than actual levels in the tests done by EPA and others.
Consequently, there is a need for a more reliable and more accurate method to measure true flow rate of a fluid through a conduit.